p62/SQSTM1 prominently accumulates in renal proximal tubules in nephropathic cystinosis

Nephropathic cystinosis, a lysosomal storage disorder, is associated with generalized proximal tubular dysfunction and progressive renal failure. The underlying molecular and cellular mechanisms leading to renal tubular injury remain largely unknown. Abnormal induction of autophagy has been shown in cystinosis. We have studied the autophagic flux in cystinosis by evaluating autophagy-specific substrates. LC3 and p62 expression was evaluated by (1) immunohistochemistry performed on kidney biopsies obtained from four nephropathic cystinosis patients, four patients with renal injury due to causes other than cystinosis, and four normal kidney tissues and (2) fluorescence imaging in cultured renal proximal tubular epithelial (RPTE) cells obtained from four nephropathic cystinosis patients and two lots of normal primary RPTE cells, both in basal and starvation conditions. p62 expression was also corroborated by western blot analysis in RPTE cells. There was a significant buildup of p62 protein in patients with nephropathic cystinosis, specifically in the proximal tubules in kidney biopsies and RPTE cells (p = 0.0004), and the accumulation was further enhanced upon starvation. Cystinotic RPTE cells exhibited a significant co-localization of p62 with LC3. Our findings indicate a potential block in the autophagic flux in cystinosis, thus providing key insights into the underlying mechanisms of tubular injury in cystinosis.